High purity phthalein derivatives and method for preparing same

ABSTRACT

The invention concerns high purity phthalcin derivatives enabling their use for medical applications or in the field of biotechnology, as well as their preparation method whereby a phthatic anhydride derivative is condensed with a naphthol or phenol derivative in an organic acid ester and the crystals of the resulting condensate are converted by action of a strong acid or one of its precursors in anhydrous medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No. PCT/FR2003/003205, filed Oct. 28, 2003, which claims the benefit of France Patent Application Serial No. 02/13,528, filed on Oct. 29, 2002. The contents of both applications are hereby incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to high purity phthaleins and to the method for preparing them. It relates more particularly to high purity fluorescein.

In the present description, the term “high purity phthalein” is intended to mean a phthalein containing at most 1% by weight, preferably at most 0.5% by weight, of impurities.

BACKGROUND OF THE INVENTION

Phthaleins are molecules having the following xanthene unit:

These products are useful as dyes in various industries, in particular the textile industry, the paper industry, printing, reprography, the food industry, the cosmetics industry and the pharmaceutical industry. Phthaleins are, currently, the subject of a considerable resurgence of interest in the health field, for their diagnostic use, in particular in the context of medical imaging and in the field of the biotechnology for labeling biological molecules (nucleic acids, proteins, lipoproteins, membrane lipids) and following the intracellular or extracellular biochemical activity of biological molecules.

For example, fluorescein is a phthalein commonly used in ophthalmology for performing retinal angiography by fluorescence. The diagnostic advantage of fluorescein angiography is currently increased by the appearance of new medicinal products for treating vascular pathologies of the retina and of the choroid and the availability of a new generation of retinographs that make it possible to perform digital fluorescence imaging that has higher performance levels and gives greater resolution than the former systems of acquisition on photographic emulsions.

In parallel, the quality and safety requirements of the new international pharmaceutical standards (ICH: International Commission of Harmonization, ICH Q Topic Q3A 1999) have considerably increased. The same is true with regard to the use of phthaleins in biotechnologies that require reagents of increasingly high quality. In order to satisfy the current requirements, the level of purity of the phthaleins used in the pharmaceutical field or that of biotechnology must necessarily be very high. By way of example, in the publication “Effective differences in the formulation of intravenous fluorescein and related side effects”, Am. J. Ophthalmol. 78, 2: 217-221, 1974, L. Yannuzzi showed a correlation between the purity of phthaleins, in particular that of fluorescein, and the tolerance of these substances when they are administered to humans by injection.

SUMMARY OF THE INVENTION

The high purity phthaleins according to the invention have the general formula (I):

in which R1, R2, R3, R4 and R5, which are identical to or different from one another, are chosen from the group comprising the following radicals or groups: hydrogen, hydroxyl, halogen, acetyl, amino, phosphate, nitro, sulfonate, carboxyl, alkylcarboxyl having from 2 to 30 carbon atoms, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, alkyloxy having from 1 to 30 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, alkyl ester having from 2 to 40 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, carboxyalkyl having from 2 to 30 carbon atoms, aminoalkyl having from 1 to 30 carbon atoms, sulfoalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, arylalkyl, haloaryl, aryl ester, succinimidyl ester, isothiocyanate, maleimide, iodoacetamide, haloacetamide, chlorosulfonic, purine or pyrimidine bases, monosaccharides, preferably hexoses or pentoses, oligosides and polyosides, polypeptides, proteins and phospholipids, R1 and R5 are not hydrogen when R1 is a group —CH₂—CH₂—COOH, R2 is a hydroxyl group and R4 is a group —COOH, these phthaleins containing no more than 1% by weight, preferably no more than 0.5% by weight, and even more preferably not more than 0.2% by weight, of residual impurities.

A phthalein that is particularly advantageous, in particular for ophthalmic applications, is fluorescein having such a purity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is known practice to prepare the phthaleins of formula (I) by the condensation of a phthalic anhydride derivative with a phenol derivative having a free ortho-position with respect to a hydroxyl group.

This condensation is carried out by heating at the melting temperature of a mixture of the phthalic anhydride and of the phenol derivative, in the desired proportions.

This condensation can optionally be carried out in a dilution solvent. It can also be carried out in the presence of a catalyst.

In the absence of solvent, during the heating, the reaction medium rapidly thickens and has a tendency to harden. Within the reaction medium, areas where the temperature is too high and areas where the temperature is not high enough then form.

In the areas where the temperature is too high, the reactants and/or the reaction product are degraded and, in the areas where the temperature is not high enough, the reaction is not complete. The product obtained is of mediocre quality since it contains by-products that are very difficult to remove.

In order to improve this known method, the addition of an inert solvent or the use of a catalyst has been considered.

Thus, U.S. Pat. No. 1,931,049 describes the addition to the reaction medium of an inert solvent consisting of a benzene-based or aliphatic hydrocarbon, more particularly ortho-dichlorobenzene. However, the condensation reaction is then incomplete, and generate intermediate products that are subsequently difficult to remove. Therefore the method described in U.S. Pat. No. 1,931,049 does not make it possible to obtain a high purity phthalein according to the definition of the present application. Moreover, higher aliphatic hydrocarbons are not miscible with the reaction medium; consequently, they introduce no improvement in terms of thermal transfer and impair the elimination of the water formed, thereby slowing down the reaction, which takes place exclusively in a hydrophobic medium.

Regarding the catalysts that are used to improve the reaction yield, these are concentrated sulfuric acid, anhydrous zinc chloride and tin chloride.

The German patent DE 360691, describes the use of an aromatic sulfonic acid as a catalyst, in particular benzenesulfonic acid, alone or combined with one of the abovementioned three catalysts.

It so happens, however, that the addition of these catalysts leads to a reaction product that sets and hardens with entrapment of impurities, which can no longer be eliminated from the desired product.

In order to eliminate both the generated by-products and the impurities, isolation and purification methods have been developed, but none have made it possible to substantially improve the quality of phthaleins.

A conventional method consists in basifying the phthalein in an aqueous medium so as to dissolve it, and then in acidifying it so as to allow it to precipitate. These two steps are repeated successively in an attempt to eliminate the impurities. However, this method does not bring about a noticeable improvement in the purity of the product since, while the impurities dissolve with the phthalein during the basifying step, they precipitate again with the phthalein during the acidification step. In addition, this purification method has the drawback of producing a considerable amount of salts which are difficult and expensive to subsequently eliminate.

U.S. Pat. No. 1,965,842 describes the purification of phthaleins, derived from hydroxybenzene, by direct extraction of the by-products with dichlorobenzene alone or in a mixture with other solvents. This direct extraction of the crude product with the solvent in question does not, however, make it possible to obtain extensive elimination of impurities, which remain partly trapped in the phthalein crystals.

None of the purification methods described in the prior art makes it possible to reach a level of purity that is sufficient to allow the pharmaceutical use of phthaleins.

SUMMARY OF THE INVENTION

Given the advantage of these molecules for medical diagnosis, the production of high purity phthaleins is a real medical need and would satisfy a high demand, in particular for pharmaceutical applications in ophthalmology, for diagnosis, in particular in medical imaging, or in the field of biotechnological applications (for example, as dye to label molecules).

The inventors have, to their credit, at the end of extensive research, found that it is possible to obtain high purity products by the condensation of a phthalic anhydride with a phenol or naphthol derivative in a specific solvent which consists of an organic acid ester.

They have also found that the use of organic acid esters as solvents makes it possible to carry out this condensation with an excellent yield, greater than 75%.

Specifically, these solvents have the particularity

-   -   firstly, of leading to specific crystallization of the phthalein         that derives from the condensation and of excluding, from the         generated crystals, all the impurities that remain dissolved in         the reaction medium, and     -   secondly, of allowing a complete condensation that is         predominant, to detrimental and unwanted side reactions, which         results in the total consumption of the reactants and minimizes         the formation of by-products.

The method in accordance with the invention thus makes it possible to obtain high purity phthaleins with a yield that is satisfactory from an industrial point of view.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a radiocrystallography spectrum of crude fluorescein.

FIG. 2 is a radiocrystallography spectrum of the fluorescein after conversion with acid (yellow).

FIG. 3 is a radiocrystallography spectrum of the crude 4′,5′-dimethyifluorescein.

FIG. 4 is a radiocrystallography of the 4′,5′-dimethyifluorescein after conversion with acid.

FIG. 5 is a radiocrystallography spectrum of the crude 4′,5′-dihydroxyfluorescein.

FIG. 6 is a radiocrystallography spectrum of the 4′,5′, dihydroxyfluorescein after conversion with acid.

DETAILED DESCRIPTION OF THE INVENTION

More particularly, the invention is related to a method for preparing phthaleins, from which the residual impurities have been removed, having general formula (I):

in which R1, R2, R3, R4 and R5, which are identical to or different from one another, are chosen from the group comprising the following radicals or groups: hydrogen, hydroxyl, halogen, acetyl, amino, phosphate, nitro, sulfonate, carboxyl, alkylcarboxyl having from 2 to 30 carbon atoms, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, alkyloxy having from 1 to 30 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, alkyl ester having from 2 to 40 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, carboxyalkyl having from 2 to 30 carbon atoms, aminoalkyl having from 1 to 30 carbon atoms, sulfoalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, arylalkyl, haloaryl, aryl ester, succinimidyl ester, isothiocyanate, maleimide, iodoacetamide, haloacetamide, chlorosulfonic, purine or pyrimidine bases, monosaccharides, preferably hexoses or pentoses, oligosides and polyosides, polypeptides, proteins and phospholipids, R1 and R5 are not representing hydrogen when R1 is a group —CH₂—CH₂—COOH, R2 is a hydroxyl group and R4 is a group —COOH, by condensation of a phthalic anhydride derivative of formula (II) with a phenol or naphthol compound of formula (III)

in which R1, R2, R3, R4 and R5 have the same meanings as above, in a solvent consisting of an organic acid ester.

Advantageously, the starting compound (III), which is condensed with the phthalic anhydride (II), is chosen from the group comprising in particular resorcinol, orcinol, naphthol, pyrogallol, alkylaminophenol and arylaminophenol.

When resorcinol is used as starting product, the method in accordance with the invention makes it possible to prepare fluorescein.

Advantageously, the solvent used in the method in accordance with the invention is an organic acid ester of formula (IV) R₆—COOR₇  (IV) in which R₆ is chosen from the group comprising the following radicals or groups: hydrogen, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, and alkyloxy having from 1 to 30 carbon atoms, R₇ representing one of the following groups: alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, or alkyloxy having from 1 to 30 carbon atoms.

Particularly advantageously, the abovementioned solvent is chosen from the group comprising methyl, ethyl, propyl or butyl benzoates, methyl, ethyl, propyl or butyl heptanoates, methyl, ethyl, propyl or butyl octanoates, methyl, ethyl, propyl or butyl laurates, methyl, ethyl, propyl or butyl myristates and methyl, ethyl, propyl or butyl palmitates, and mixtures thereof.

The solvent is chosen according to its boiling point, so as to make it possible to carry out the condensation reaction at a temperature of between 150° C. and 250° C.

The condensation reaction can be carried out at atmospheric pressure or under a pressure that is adjusted according to the difference that exists between the temperature corresponding to the boiling point of the solvent and the temperature necessary to carry out the reaction, in particular under a pressure greater than atmospheric pressure when the boiling point of the compound (IV) is lower than the reaction temperature.

The condensation reaction can be carried out in the presence of a catalyst chosen from the group comprising in particular Lewis acids, such as ZnCl₂ or AlCl₃, Brönsted acids such as H₂SO₄, polyphosphoric acid, or salts thereof.

Advantageously, the catalyst used is an alkali metal salt of hydrogen sulfate. Particularly advantageously, the catalyst used is potassium hydrogen sulfate (KHSO₄) or sodium hydrogen sulfate. The use of hydrogen sulfate as catalyst makes it possible to obtain an excellent yield for the condensation reaction and has the advantage, unlike other catalysts, of making it possible to obtain complete condensation of the reactants, of being able to be completely eliminated from the phthalein obtained, and of not inducing the formation of tars in the reaction medium.

At the end of the condensation reaction, a crude product is obtained, the organic purity of which is already much higher than that of the products obtained with the methods of the prior art, since it is equal to or greater than 95%.

However, this purity is not yet sufficient to allow pharmaceutical use, in particular by injection. Furthermore, as shown above, the purification methods described in the prior art do not result in a process that makes it possible to notably improve this purity.

At the end of extensive research, surprisingly and unexpectedly, the inventors have found a method that considerably increases the purity of crude phthaleins resulting from the condensation reaction, by treating them with a strong acid or one of its precursors in an anhydrous organic medium.

According to an advantageous embodiment, the method in accordance with the invention consequently comprises, after the condensation reaction, a step consisting in acidifying the product resulting from the condensation, in an anhydrous organic medium, by addition of a strong acid or one of its precursors, chosen from the group comprising in particular sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid, pyrophosphate (P₂O₅), and mixtures thereof. This acidification is carried out until the phthalein crystals resulting from the condensation are converted to phthalein crystals having a different structure.

The effect of this acidification step is to convert the crude phthalein that is insoluble and dispersed in the organic medium into a form that has a different crystalline structure and is slightly soluble in this same medium. This conversion is very rapid, and takes place via an intermediate solubilization phase during which the impurities are released and completely eliminated from the phthalein crystals. This new purification method is very advantageous since it requires very little solvent and yields to a very high purity in a very short time.

According to the method of the present invention, this purification step is carried out by dispersing the crude phthalein resulting from the condensation, in an anhydrous solvent, preferably in an alcohol, a ketone, an ether or a halogenated solvent, that is either used alone or as a mixture, even more preferably in absolute ethanol or acetone, alone or as a mixture.

The dispersion of crude phthalein thus obtained is acidified by the addition of a strong acid or one of its precursors, chosen from the group comprising in particular sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid, pyrophosphate (P₂O₅), and mixtures thereof.

According to a particularly advantageous embodiment, the acidification is carried out either by sparging gaseous hydrochloric acid into the phthalein dispersion, or by adding hydrochloric acid dissolved beforehand in an organic solvent.

According to another advantageous embodiment of the method in accordance with the invention, the product obtained after acidification is washed with a washing solution chosen from the group comprising water, polar solvents such as alcohols or ketones, or slightly polar solvents such as ethers and halogenated solvents, used pure or as a mixture.

At the end of the purification step, the phthaleins thus treated have a purity of greater than 98%, preferably greater than 99%, more preferably greater than 99.5%, and even more preferably of 99.8%.

The phthaleins thus obtained have the suitable quality to prepare other phthaleins of formula (I) by chemical modification of the groups R1, R2, R3, R4 and R5 according to the conventional methods of the art, in particular to obtain phthaleins that can be used for labeling biological molecules (nucleic acids, proteins, lipoproteins, membrane lipids) and in the field of biotechnological applications (for example, for the labeling of molecules and of their intracellular or extracellular biochemical activity).

The invention is particularly suitable for preparing very high purity fluorescein, i.e. such that its content of each of the by-products of the reaction is less than or equal to 0.2%, and preferably less than or equal to 0.1%, the sum of the contents of each of these by-products being less than or equal to 0.5%.

The method for preparing a fluorescein having the abovementioned purity comprises the following successive steps:

-   -   condensing phthalic anhydride with resorcinol, in a solvent         which is an ester of an aliphatic or aromatic organic acid,         preferably methyl benzoate, in the presence of a catalyst chosen         from the group of Brönsted acids,     -   suspending the red-colored crystals obtained in the preceding         step in an anhydrous solvent chosen from the group comprising         alcohols such as absolute ethanol, ketones such as acetone,         ethers, halogenated solvents, or mixtures thereof,     -   acidifying the suspension obtained, by the addition of a strong         acid or one of its precursors, chosen from the group comprising         sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric         acid, hydriodic acid, polyphosphoric acid, pyrophosphate (P₂O₅),         or mixtures thereof, until yellow-colored crystals are obtained,     -   washing the crystals thus obtained with a washing solution         chosen from the group comprising water, alcohols, ketones,         ethers and halogenated solvents, pure or as a mixture, until         red-colored crystals are obtained.

An X-ray diffraction analysis of the red-colored fluorescein crystals is represented in FIG. 1; an X-ray diffraction analysis of the yellow-colored fluorescein crystals is represented in FIG. 2; these X-ray-diffraction analysis being obtained on the following equipment: Philips 1729 generator, Philips 1050 goniometer, Cu Kα radiation, Gonio acquisition software and Rayon processing software, under the following operating conditions:

Voltage 40 mV Intensity 40 mA Number of points 4000 Number of passages 10 Acquisition period 250 ms Start angle (°θ) 3.000 End angle (°θ) 23.000 Standard silicon

According to an advantageous embodiment of the method for preparing fluorescein, the catalyst used for the condensation reaction is the hydrogen sulfate of an alkali metal, preferably potassium hydrogen sulfate or sodium hydrogen sulfate.

According to another advantageous embodiment of the method for preparing fluorescein, the acidification is carried out by sparging gaseous hydrochloric acid into the fluorescein suspension or by the action, on this fluorescein, of hydrochloric acid in solution in an anhydrous organic solvent, preferably chosen from the group comprising alcohols, ketones, ethers and halogenated solvents, used alone or as a mixture, even more preferably isopropanol, absolute ethanol or acetone, pure or as a mixture.

Advantageously, the washing step is carried out with a mixture of water, of ethanol and of acetone.

By means of this method in accordance with the invention, it is possible to prepare fluorescein having a purity of greater than or equal to 99.7%, which provides unquestionable advantages, in particular for pharmaceutical uses in diagnosis, especially in medical imaging or else in the field of biotechnological applications.

The inventors have demonstrated a novel crystallographic form of fluorescein being yellow-colored crystals. This novel crystallographic form is identified by the X-ray diffraction analysis given in FIG. 2, which was determined under the conditions mentioned above.

They have also demonstrated a novel crystallographic form of the following compounds: 4′,5′-dihydroxyfluorescein and 4′,5′-dimethylfluorescein.

Thus, the invention relates to the yellow-colored fluorescein crystals for which the X-ray diffraction analysis is given in FIG. 2.

The invention also relates to the yellow-colored 4′,5′-dihydroxyfluorescein crystals for which the X-ray diffraction analysis is given in FIG. 4.

It also relates to the dark red-colored 4′,5′-dimethylfluorescein crystals for which the X-ray diffraction analysis is given in FIG. 6.

All these spectra were determined with the equipment and under the conditions mentioned above.

The invention will be described in more detail with the aid of the following examples which are not limiting but relate to advantageous embodiments.

EXAMPLES Example 1 Preparation of High Purity Fluorescein Synthesis of Fluorescein

A mixture comprising 55 g of resorcinol, 30 g of phthalic anhydride, 2 g of potassium hydrogen sulfate and 500 ml of methyl benzoate is brought to 200° C. for 6 hours. After cooling, the red crystals of crude fluorescein are washed with acetone and dried.

Mass obtained=51.8 g (78%).

These crystals are analyzed by X-ray crystallography under the following operating conditions:

Equipment

Philips 1729 generator

Philips 1050 goniometer

Cu Kα radiation

Gonio acquisition software

Rayon processing software

Conditions Voltage 40 mV Intensity 40 mA Number of points 4000 Number of passages 10 Acquisition period 250 ms Start angle (°θ) 3.000 End angle (°θ) 23.000 Standard silicon

The radiocrystallogram of FIG. 1 is obtained, for which the peaks are identified below:

Theta % Number (degrees) Distance (Å) Intensities of counts 5.3600 8.2457 18.09 647 5.9500 7.4306 31.10 1112 6.7000 6.6020 36.30 1298 8.3750 5.2883 45.95 1643 9.1050 4.8675 10.79 386 9.4300 4.7012 14.09 504 9.9950 4.4379 8.03 287 10.9150 4.0678 9.73 348 11.6200 3.8241 16.41 587 11.7300 3.7888 5.40 193 12.4200 3.5813 2.82 101 13.2150 3.3694 100.00 3576 13.5050 3.2983 7.47 267 13.8850 3.2097 79.03 2826 14.9650 2.9828 5.62 201 15.3100 2.9172 3.91 140 15.7600 2.8359 11.69 418 16.0000 2.7945 8.67 310 16.9100 2.6481 4.28 153 17.4750 2.5650 3.97 142 17.8550 2.5122 3.80 136 18.2300 2.4622 2.52 90 18.5150 2.4256 2.96 106 18.8250 2.3871 5.03 180 19.0800 2.3563 3.27 117 20.4300 2.2066 3.05 109 20.6800 2.1811 4.53 162 20.9300 2.1562 3.08 110 21.1800 2.1319 2.38 85 22.9400 1.9762 4.53 162

Wavelength: 1.54051 Å

Purification of Crude Fluorescein

50 g of crude fluorescein obtained in the previous step are stirred into 1000 ml of ethanol/acetone mixture. Concentrated sulfuric acid is added to this mixture, until complete conversion of the red crystals to yellow crystals is obtained.

The yellow crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of FIG. 2 is obtained, for which the peaks are identified below:

Theta Distances % Number of (degrees) (Å) Intensities counts 3.3500 13.1813 8.55 56 4.0050 11.0283 47.18 309 4.8550 9.1010 12.98 85 5.2650 8.3940 9.01 59 5.4500 8.1099 11.45 75 5.7900 7.6352 52.98 347 6.3000 7.0193 16.95 111 6.7700 6.5340 17.86 117 7.4900 5.9090 17.56 115 8.0300 5.5140 69.77 457 8.4250 5.2572 33.13 217 8.7800 5.0462 8.40 55 9.1550 4.8411 15.88 104 9.3400 4.7461 42.44 278 9.6550 4.5926 34.66 227 9.8800 4.4891 24.43 160 10.1950 4.3518 57.71 378 10.5800 4.1951 58.02 380 10.7350 4.1352 38.17 250 11.1650 3.9779 28.85 189 11.4800 3.8701 30.23 198 11.7200 3.7919 59.85 392 12.1100 3.6716 26.41 173 12.3950 3.5884 58.47 383 12.6400 3.5200 100.00 655 12.9450 3.4384 30.84 202 13.1550 3.3845 56.64 371 13.8350 3.2211 14.66 96 14.0550 3.1717 24.12 158 14.3550 3.1068 36.34 238 14.6700 3.0415 42.90 281 15.1300 2.9511 42.90 281 15.7500 2.8377 18.47 121 15.9400 2.8047 18.17 119 16.2500 2.7526 15.42 101 16.7550 2.6719 17.71 116 16.8500 2.6573 19.39 127 17.2250 2.6011 18.17 119 17.7300 2.5293 15.73 103 18.4200 2.4377 10.23 67 18.5750 2.4180 10.23 67 19.2650 2.3345 15.88 104 19.5800 2.2984 10.84 71 19.8000 2.2739 11.91 78 20.1150 2.2397 13.28 87 20.6500 2.1841 14.81 97 21.1500 2.1348 11.30 74 21.2750 2.1228 10.08 66 21.8100 2.0732 7.94 52 22.2500 2.0342 9.16 60 22.6250 2.0022 9.62 63 22.9100 1.9786 9.01 59

Wavelength: 1.54051 Å

These crystals are filtered off and then washed with an ethanol/acetone/water (40/40/20) mixture. The washing turns the yellow fluorescein crystals to red fluorescein crystals, which have a higher purity.

Purity by HPLC: 99.8%

Example 2 Preparation of high purity 4′,5′-dimethylfluorescein Synthesis of 4′,5′-dimethylfluorescein

A mixture comprising 62 g of 2-methylresorcinol, 30 g of phthalic anhydride, 2 g of potassium hydrogen sulfate and 500 ml of ethyl laurate is brought to 200° C. for 3 hours. After cooling, the crude product is filtered and washed with acetone and then dried. The product obtained is a dark orange powder.

Mass obtained=49.7 g (69%)

These crystals are analyzed by X-ray crystallography in the same way as in example 1. The X-ray diffraction analysis of FIG. 3 is obtained, for which the peaks are identified below.

Theta Distances % Number of (degrees) (Å) Intensities counts 3.5700 12.3700 7.38 81 5.0290 8.7938 45.49 499 6.6650 6.6365 62.26 683 6.7400 6.5630 60.07 659 7.1300 6.2057 87.24 957 7.4550 5.9366 20.97 230 8.3700 5.2915 6.56 72 8.9300 4.9621 30.63 336 9.0950 4.8728 10.12 111 9.3300 4.7511 21.24 233 9.4550 4.6889 19.05 209 10.0350 4.4204 12.12 133 10.3250 4.2975 18.87 207 10.6000 4.1873 11.49 126 10.8300 4.0994 39.65 435 11.2900 3.9344 12.49 137 11.4500 3.8801 10.85 119 11.7550 3.7808 28.17 309 11.9050 3.7339 52.42 575 12.2550 3.6288 79.95 877 12.7050 3.5023 8.11 89 13.2400 3.3631 21.15 232 13.4200 3.3188 100.00 1097 13.7100 3.2499 10.85 119 14.5250 3.0712 8.84 97 14.8350 3.0084 23.25 255 15.0550 2.9654 17.59 193 15.1550 2.9463 15.13 166 15.3650 2.9070 25.80 283 15.7050 2.8456 20.51 225 16.1900 2.7625 14.13 155 16.7550 2.6719 9.02 99 17.5400 2.5558 7.47 82 17.8850 2.5081 8.75 96 18.1050 2.4786 6.75 74 18.4900 2.4288 19.51 214 18.7950 2.3907 6.11 67 18.8900 2.3791 6.84 75 19.6800 2.2872 15.50 170 20.2100 2.2296 7.38 81 20.3950 2.2103 9.66 106 20.6800 2.1811 6.84 75 20.9950 2.1498 5.38 59 21.4650 2.1049 7.11 78 21.6500 2.0878 8.84 97 21.9350 2.0620 9.21 101 22.1250 2.0451 9.02 99 22.5300 2.0102 5.47 60 22.7850 1.9889 5.93 65

Wavelength: 1.54051 Å

Purification of 4′,5′-dimethylfluorescein

40 g of crude 4′,5′-dimethylfluorescein are added to 800 ml of ethanol/acetone mixture. Concentrated sulfuric acid is added to this mixture, until complete conversion of dark orange-colored crystals to yellow-colored crystals is obtained.

The yellow crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of FIG. 4 is obtained, for which the peaks are identified below.

Theta Distances % Number of (degrees) (Å) Intensities counts 3.7050 11.9199 22.22 172 4.6150 9.5732 45.61 353 4.9300 8.9629 46.77 362 6.3300 6.9861 8.53 66 6.6150 6.6864 6.20 48 7.3000 6.0619 19.51 151 7.4500 5.9405 32.56 252 7.6500 5.7861 26.61 206 8.1250 5.4499 64.21 497 8.6300 5.1332 22.48 174 9.5950 4.6211 6.59 51 10.4450 4.2487 41.34 320 10.8500 4.0919 10.21 79 11.3550 3.9122 14.60 113 11.7000 3.7983 9.17 71 11.7950 3.7682 9.30 72 12.2250 3.6375 23.13 179 12.4500 3.5728 58.79 455 12.7650 3.4861 19.90 154 12.8950 3.4515 18.60 144 13.1000 3.3984 100.00 774 13.6350 3.2675 67.44 522 14.1400 3.1530 30.75 238 14.5250 3.0712 14.08 109 14.8400 3.0074 22.74 176 15.1750 2.9425 19.38 150 15.3500 2.9098 30.23 234 15.6950 2.8474 20.93 162 16.0950 2.7784 7.24 56 16.2850 2.7468 7.49 58 16.5950 2.6969 8.01 62 16.9100 2.6481 14.08 109 17.3200 2.5873 8.66 67 17.5400 2.5558 7.88 61 17.9150 2.5040 13.31 103 18.3550 2.4460 9.69 75 18.4800 2.4300 10.59 82 18.8600 2.3828 15.37 119 19.2350 2.3380 9.43 73 19.4850 2.3092 7.75 60 20.2100 2.2296 14.99 116 20.7100 2.1781 7.11 55 20.8350 2.1656 6.59 51 21.0850 2.1411 6.33 49 21.5250 2.0993 12.92 100 21.6500 2.0878 8.79 68 21.8400 2.0705 6.98 54 22.0900 2.0482 10.34 80 226550 1.9997 7.11 55

Wavelength: 1.54051 Å

After filtration and recrystallization from an acetone/water mixture or washing in ethanol/acetone/water, the yellow-colored crystals turn to dark orange-colored crystals.

Example 3 Preparation of 4′,5′-dihydroxyfluorescein Synthesis of 4′,5′-dihydroxyfluorescein

A mixture comprising 63 g of pyrogallol, 30 g of phthalic anhydride, 2 g of potassium hydrogen sulfate and 500 ml of ethyl myristate is brought to 200° C. for 3 hours. After cooling, the crude product is filtered and washed with acetone and then dried. The product obtained is a grayish-brown- or anthracite-colored powder.

Mass obtained=43.5 g (59.7%)

The crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of FIG. 5 is obtained, the peaks of which are identified below.

Theta Distances % Number (degrees) (Å) Intensities of counts 3.6450 12.1158 73.08 980 5.9200 7.4681 8.13 109 6.7200 6.5824 13.35 179 7.3350 6.0331 10.29 138 8.5050 5.2081 11.78 158 9.2300 4.8021 15.59 209 9.7400 4.5529 18.20 244 10.3400 4.2914 21.40 287 10.9150 4.0678 6.71 90 11.2600 3.9447 8.80 118 12.2550 3.6288 18.64 250 12.5550 3.5434 12.75 171 13.2000 3.3731 11.86 159 13.7200 3.2476 100.00 1341 14.7150 3.0324 17.75 238 14.8600 3.0034 13.57 182 16.1600 2.7675 6.79 91 17.2550 2.5967 7.83 105 18.3550 2.4460 6.86 92

Wavelenth: 1.54051 Å

Purification of 4′,5′-dihydroxyfluorescein

40 g of crude 4′,5′-dihydroxyfluorescein are added to 800 ml of ethanol/acetone mixture. Concentrated sulfuric acid is added to this mixture, until complete conversion of grayish-brown- or anthracite-colored crystals to reddish-brown- or mahogany-colored crystals is obtained.

The crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of FIG. 6 is obtained, for which the peaks are identified below:

Theta Distances % Number of (degrees) (Å) Intensities counts 3.2500 13.5865 90.53 526 3.5350 12.4923 18.76 109 3.7850 11.6683 11.70 68 3.9450 11.1958 13.25 77 4.1650 10.6053 15.15 88 4.5400 9.7310 10.33 60 4.6350 9.5319 10.15 59 6.3950 6.9154 9.29 54 6.5500 6.7525 11.53 67 6.8200 6.4863 27.71 161 7.1150 6.2187 10.50 61 7.4800 5.9168 21.51 125 7.8700 5.6253 12.91 75 8.2200 5.3874 27.88 162 8.7800 5.0462 41.48 241 9.2200 4.8073 10.15 59 9.5350 4.6499 11.70 68 9.7950 4.5276 42.17 245 10.1000 4.3923 17.21 100 10.5400 4.2108 16.70 97 10.8850 4.0789 16.52 96 11.6500 3.8144 25.13 146 11.8900 3.7385 13.60 79 12.0450 3.6911 13.08 76 12.3900 3.5898 16.52 96 12.5800 3.5365 14.80 86 12.8750 3.4568 27.37 159 12.9950 3.4254 25.82 150 13.2900 3.3507 67.64 393 13.5750 3.2816 27.02 157 13.7150 3.2488 27.02 157 14.1100 3.1596 100.00 581 14.6200 3.0516 15.66 91 14.7750 3.0203 18.24 106 15.0400 2.9683 25.13 146 15.3050 2.9181 24.78 144 15.6850 2.8491 16.35 95 15.9400 2.8047 11.02 64 16.2500 2.7526 10.67 62 16.5950 2.6969 12.22 71 17.1650 2.6099 9.81 57 17.5700 2.5516 8.78 51 17.7600 2.5252 9.81 57 18.0750 2.4826 11.36 66 18.5150 2.4256 8.43 49 18.7650 2.3944 13.08 76 19.0450 2.3605 11.36 66 20.1950 2.2312 50.09 291 20.6150 2.1877 9.12 53 21.0550 2.1440 12.56 73 21.4650 2.1049 9.29 54 21.6850 2.0846 9.98 58 22.0600 2.0509 9.81 57

Wavelength: 1.54051 Å

After filtration and washing in water, the reddish-brown- or mahogany-colored crystals turn to grayish-brown- or anthracite-colored crystals. 

1. A method for preparing phthaleins, wherein the residual impurities have been removed, having the general formula (I):

wherein R₁, R₂, R₃, R₄ and R₅, which are identical to or different from one another, are selected from the group consisting of hydrogen, hydroxyl, halogen, acetyl, amino, phosphate, nitro, sulfonate, carboxyl, alkylcarboxyl having from 2 to 30 carbon atoms, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, alkyloxy having from 1 to 30 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, alkyl ester having from 2 to 40 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, carboxyalkyl having from 2 to 30 carbon atoms, aminoalkyl having from 1 to 30 carbon atoms, sulfoalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, arylalkyl, haloaryl, aryl ester, succinimidyl ester, isothiocyanate, maleimide, iodoacetamide, haloacetamide, chlorosulfonic, purine or pyrimidine bases, monosaccharides, oligosides and polyosides, polypeptides, proteins and phospholipids, R₃ and R₅ are not hydrogen when R₁ is a group —CH₂-CH₂—COOH, R₂ is a hydroxyl group and R₄ is a group —COCH, wherein a phthalic anhydride derivative of formula (II) is condensed with a phenol or naphthol compound of formula (III)

in which R₁, R₂, R₃, R₄ and R₅ have the same meanings as above, the condensation being carried out in a solvent consisting of an organic acid ester of formula IV R₆ —COOR₇  (IV) wherein R₆ is selected from the group consisting of hydrogen, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, and alkyloxy having from 1 to 30 carbon atoms, R₇ being selected from the group consisting of alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, or alkyloxy having from 1 to 30 carbon atoms.
 2. The method as claimed in claim 1, wherein the compound of formula (Ill) is selected from the group consisting of resorcinol, orcinol, naphthol, pyrogallol, alkylaminophenol and arylaminophenol.
 3. The method as claimed in claim 1, wherein the organic acid ester is selected from the group consisting of methyl benzoate, ethyl benzoate, propyl benzoate, butyl benzoate, methyl heptanoate, ethyl heptanoate, propyl heptanoate, butyl heptanoate, methyl octanoate, ethyl octanoate, propyl octanoate, butyl octanoate, methyl laurate, ethyl laurate, propyl laurate, butyl laurate, methyl myristate, ethyl myristate, propyl myristate, butyl myristate, methyl palmitate, ethyl palmitate, propyl palmitate, butyl palmitate, and mixtures thereof.
 4. The method as claimed in claim 1, wherein the condensation reaction is carried out at between 150° C. and 250° C., optionally under pressure.
 5. The method as claimed in claim 1, wherein the reaction is carried out in the presence of a catalyst selected from the group consisting of Lewis acids, such as ZnCI₂ or AICI_(3,) Brödnsted acids such as H₂SO₄ or polyphosphoric acid.
 6. The method as claimed in claim 5, wherein the catalyst is an alkali metal hydrogen sulfate.
 7. The method as claimed in claim 6, wherein the catalyst is potassium hydrogen sulfate (KHSO₄) or sodium hydrogen sulfate (NaHSO₄ ).
 8. A method for acidifying the product resulting from the condensation of a phthalic anhydride derivative of formula (II) with a phenol or naphthol compound of formula (Ill), the formulae (II) and (III) being those of claim 1, wherein the reaction is carried out in an anhydrous organic medium, by the addition of a strong acid or one of its precursors, selected from the group consisting of sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid, pyrophosphate (P₂O₅), and mixtures thereof, the acidification being carried out until the phthalein crystals resulting from the condensation are converted to phthalein crystals having a different structure.
 9. The method as claimed in claim 8, comprising a step consisting in washing the product obtained after acidification, said washing step being carried out with a washing solution selected from the group consisting of water, alcohols, ketones, ethers and halogenated solvents, pure or as a mixture, until the crystals are reconverted to the structure that they had before the acidification reaction.
 10. A method for preparing a fluorescein having a purity such that its content of each of the by-products of the reaction is less than or equal to 0.2%, the sum of the contents of each of these by-products being less than or equal to 0.5%, said method comprising the following successive steps: condensing phthalic anhydride with resorcinol, in a solvent consisting of an ester of an aliphatic or aromatic organic acid, in the presence of a catalyst selected from the group consisting of Lewis acids or Brönsted acids, said ester of an aliphatic or aromatic organic acid having the formula IV R₆—COOR₇  (IV) wherein R₆ is selected from the group consisting of hydrogen, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, and alkyloxy having from 1 to 30 carbon atoms, R₇ being selected from the group consisting of alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, or alkyloxy having from 1 to 30 carbon atoms, suspending the red-colored crystals obtained in the preceding step in an anhydrous solvent selected from the group consisting of alcohols such as absolute ethanol, ketones such as acetone, ethers, halogenated solvents, or mixtures thereof, acidifying the suspension thus obtained by the addition of a strong acid or one of its precursors, selected from the group consisting of sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid pyrophosphate (P₂O₅), and mixtures thereof, until the red-colored crystals are converted to yellow-colored crystals, washing the crystals obtained with a washing solution selected from the group consisting of water, alcohols, ketones, ethers and halogenated solvents, pure or as a mixture, this washing being continued until the yellow-colored crystals are reconverted to red-colored crystals.
 11. The method for preparing a fluorescein as claimed in claim 10, having a purity such that its content of each of the by-products of the reaction is less than or equal to 0.1%.
 12. The method for preparing a fluorescein as claimed in claim 10, wherein the solvent used in the condensation reaction is the ethyl or methyl benzoate or ethyl or methyl palmitate.
 13. The method for preparing a fluorescein as claimed in claim 10, wherein the catalyst used for the condensation reaction is an alkali metal hydrogen sulfate.
 14. The method for preparing a fluorescein as claimed in claim 13, wherein the catalyst is potassium hydrogen sulfate or sodium hydrogen sulfate.
 15. The method as claimed in claim 8, wherein the acidification is carried out by addition of hydrochloric acid into the phthalein suspension in an anhydrous organic solvent.
 16. The method of claim 15, wherein the addition of hydrochloric acid is effected by sparging gaseous hydrochloric acid Into the phthalein suspension.
 17. The method of claim 15, wherein the anhydrous organic solvent is an alcohol, ketone, ether, halogenated solvent, or a mixture thereof.
 18. The method of claim 17, wherein the anhydrous organic solvent is isopropanol, absolute ethanol, acetone, or a mixture thereof.
 19. The method of claim 10, wherein the acidification Is carried out by addition of hydrochloric acid Into the fluorescein suspension In an anhydrous organic solvent.
 20. The method of claim 19, wherein the addition of hydrochloric acid is effected by sparging gaseous hydrochloric acid into the fluorescein suspension.
 21. The method of claim 19, wherein the hydrous organic solvent is an alcohol, ketone, ether, halogenated solvent, or a mixture thereof.
 22. The method of claim 21, wherein the anhydrous organic solvent is isopropanol, absolute ethanol, acetone, or a mixture thereof. 